It is generally known that in air-fuel ratio control of a vehicle engine, higher efficiency of control is achieved by learning control parameters.
Tokkai Sho 64-19143 published by the Japanese Patent Office in 1989 discloses a plurality of learning values which are introduced into air-fuel ratio control. An air-fuel ratio controller according to this prior art determines an injection amount Ti by the following expressions (1) and (2). Ti is calculated as an injection pulse width of a fuel injector. ##EQU1## where, Tp=Basic injection pulse width, Qs=Intake air flowrate,
n=Engine rotation speed, PA1 KCONST=Constant, PA1 LqfKLCD1=Learning value based on flowrate characteristics of a fuel injector. EQU Ti=Tp.multidot.COEF.multidot.KLCD2.multidot.ALPHA+Ts+KLCD3 (2) PA1 ALPHA=air-fuel ratio feedback correction coefficient, PA1 Ts=ineffectual pulse width, PA1 KLCD2=learning value of ALPHA, PA1 KLCD3=learning value of ineffectual pulse width.
where, COEF=sum of 1 and various correction coefficients,
Here, KLCD1 and COEF are omitted in the equations (1) and (2) in order to simplify the discussion, and the above calculation equations are rewritten by the next expressions (3), (4). EQU Tp=(Qs/n).multidot.KCONST (3) EQU Ti=Tp.multidot.KLCD2.multidot.ALPHA+Ts+KLCD3 (4)
The ineffectual pulse width Ts is an item which compensates for response delay of the fuel injector. Even when a command signal is given to open the fuel injector for a time corresponding to Tp, the injector does not open immediately due to a response delay of the injector.
The fuel injection period is shortened by this response delay, and the fuel injection amount is correspondingly deficient. The effective injection time of the fuel injector is therefore made equal to Tp by increasing the time by Ts. The response delay of the fuel injector is normally considerably affected by the battery voltage, and the response delay increases the more the battery voltage falls. Ts is therefore set to a larger value the more the battery voltage falls.
On the other hand, the air-fuel ratio deviates from the stoichiometric air-fuel ratio when there are errors or time-dependent deterioration in the performance of the air flow meter and flowrate characteristics of the fuel injector. The deviation of the true air-fuel ratio from the stoichiometric air-fuel ratio in this case is referred to as a steady state deviation. KLCD2 has the function of canceling steady-state deviation under running conditions in which air-fuel ratio feedback control is not performed so that this control is not affected by error and time-dependent deterioration of air-fuel ratio control components.
For example, in the case where the fuel injection performance of the injector is lower than specified, the fuel injection amount is deficient and the air-fuel ratio tends towards lean. In order to restore this lean air-fuel ratio to the stoichiometric air-fuel ratio, ALPHA takes a value bigger than 100%, where 100% is the center value of ALPHA.
KLCD2 is updated based on the value of ALPHA and KLCD2 is also a coefficient having a center value of 100%, so KLCD2 will vary to a larger value than 100%. Hence when KLCD2 converges, the air-fuel ratio settles down to the stoichiometric air-fuel ratio.
Therefore even if the fuel injection performance of the injector is lower than specified, the apparent fuel injection performance of the fuel injector is equal to the specified performance.
Similarly KLCD3 is a value for eliminating the effect of error and time-dependent deterioration exerted on fuel injector opening timing.
For example, when the injector opening timing is delayed relative to the design timing, the injection amount is deficient and the air-fuel ratio of the exhaust is lean, because the delay in opening of the fuel injector results in a shorter duration of fuel injection.
To restore this lean air-fuel ratio to the stoichiometric air-fuel ratio, ALPHA varies to a value larger than 100%, and KLCD3 which is updated based on ALPHA varies to a larger value than 0 milliseconds. The center value of KLCD3 in this case is 0 milliseconds.
Hence, the air-fuel ratio settles down to the stoichiometric air-fuel ratio when KLCD3 converges.
Therefore even if the fuel injector opening timing is delayed relative to the design timing, the apparent fuel injection performance of the fuel injector is equal to the specified performance.
In the aforesaid prior apparatus, learning permission conditions were different for KLCD2 and KLCD3. In other words, whereas KLCD2 is updated for each learning region, KLCD3 is updated only in the low load area. In the low load area, the basic injection pulse width Tp is small relative to the ineffectual pulse width Ts. Therefore, the proportion of Ts error in the deviation of the air-fuel ratio from the stoichiometric air-fuel ratio in the low load region is high, and this is suitable for updating KLCD3 which is related to Ts. However in order to suppress the adverse effect of incorrect learning, i.e., a large air-fuel ratio error, it is desirable to limit each of the learning values mentioned above to within predetermined limits.
However, whereas the above-mentioned learning value KLCD1 and KLCD2 which are learning values introduced in the form of a multiplication (referred to hereafter as a multiplication term), KLCD3 is a learning value introduced in the form of an addition (referred to hereafter as an addition term). Hence, setting a limitation independently on every learning value in a calculation equation which comprises a plurality of learning values of different type, does not necessarily limit the total effect of the learning values within a desirable range.
In other words, when all the learning values approach their upper or lower limit, the total effect of these learning values may have already been out of the desirable range.
This will be described more specifically. Assume that the following limits have been set regarding KLCD2 (learning value of multiplication term) and KLCD3 (learning value of addition term) in the equation (4). EQU 90%.ltoreq.KLCD2.ltoreq.110% EQU -0.1 msec.ltoreq.KLCD3 &lt;+0.1 msec
KLCD2 is a multiplication term for the basic injection pulse width Tp, and KLCD3 is an addition term for the basic injection pulse width Tp.
Therefore the upper limit of KLCD2 is 110% and the lower limit of KLCD2 is 90% regardless of the magnitude of Tp. On the other hand, the upper and lower limits of KLCD3 become relatively large as Tp becomes smaller.
FIG. 15A is a diagram which shows each limiting value of the learning values KLCD2 and KLCD3 as percentages relative to the basic injection pulse width Tp. As is clear from this diagram, in the region where Tp is small, the proportion of Tp represented by the limiting value of the learning value KLCD3 remarkably increases. Therefore if the learning value KLCD3 is in the vicinity of the limit of .+-.0.1 msec when there is a learning value KLCD2 in the vicinity of the upper limit 110% or the lower limit 90%, the proportion occupied by the two learning values may exceed .+-.10% of Tp.
However, when the proportion of the learning value in Tp increases, the effect on incorrect learning on the air-fuel ratio fluctuation also becomes large. From the viewpoint of stable running of the engine, it is not desirable that the air-fuel ratio fluctuates due to incorrect learning.